Whooping cough might sound like an old-time disease of yore, like polio or typhoid, but it’s alive and all too well. The United States is currently in the middle of one of the largest outbreaks in 50 years. Illinois, Iowa, Washington, California, Montana, and several other states are struggling to stop the disease from spreading. And with the new school year underway, there’s concern that classrooms could become incubators.

Though the roots of these types of outbreaks are generally traced back to vaccine deniers (folks who refuse to vaccinate their children), this particular one is likely to be the fault of the vaccine wearing off sooner than anticipated.

Whooping cough (also known as pertussis) is a bacterial disease that causes people infected to cough violently and uncontrollably. The coughing can become so severe that it forces all the air out of a sick person’s lungs, causing them to gasp for air and make a whooping sound. The first whooping cough vaccine was released in the 1940s. At the time, the United States was seeing around 200,000 cases every year. Since then the rates have dropped significantly.

The number of whooping cough cases in the country cycles every three to five years, says Dr. Tom Clark, a medical epidemiologist with the Centers for Disease Control and Prevention who specializes in whooping cough. “A lot of places will have outbreaks, and each time we see a higher peak than the last time. In 2005 there were 25,000 cases and in 2010 there were over 27,000.”

The normal ebb and flow, however, is different this year. According to Clark, there have already been 22,000 cases and, because there are several months left in 2012, “we’re on track for not just a record but probably a big record.” In addition, it’s unusual to have such a large number of cases this soon after the previous peak.

So what is the cause of this unusual uptick in whooping cough cases? The culprit, according to a study just released in the New England Journal of Medicine, may be the newer version of whooping cough vaccine called DTaP (diphtheria, tetanus and pertussis), administered in five shots. Though the duration of the whooping cough vaccine was previously unknown, the researchers behind the study at the Kaiser Permanente Vaccine Study Center in Oakland, California, found that after the fifth and final DTaP shot was administered to 277 children aged 4 to 12, their chances of getting whooping cough increased 42 percent per year.

According to Clark, the previous, and longer lasting, whooping cough vaccine called DTP was phased out in the 1990s after concerns about reactions at the injection site. About 40 to 50 percent of patients were experiencing pain and swelling at the vaccination site. Additionally, there were rare reports of persistent neurological problems. And, though the correlation between the whooping cough vaccine and the neurological conditions were never proven, people began to lose confidence in the whooping cough vaccine. Clark says that drug companies stopped producing the questionable vaccine, and as it was phased out a new vaccine—the one administered today—became the standard.

But the new pertussis vaccine seems to be wearing off sooner than expected. Though it’s not entirely known why the new whooping cough vaccine wanes in effectiveness more quickly than the previous one (all vaccines lose efficacy over time, which is why nearly every vaccine has recommended booster shots) James Cherry, a specialist in pediatric infectious diseases at UCLA, suspects the reason is that the new whooping cough vaccine does not contain as many the disease’s proteins.

The previous vaccine contained 1,200 different types of proteins that make up pertussis. During the 1980s, Cherry says, studies found that some of those proteins were more active in helping protect people from the disease than others. As a result, the new whooping cough vaccines contain only the pertussis proteins thought to be the most important.

Unfortunately, he adds, “Some of that information was wrong,” meaning the two vaccines used most commonly today, one of which has three proteins and the other has five, are likely less effective because they're missing elements that could help the immune system fight the disease.

That’s not to say the new vaccines aren’t helping at all—they are protecting us from pertussis, just not for as long as expected. According to Cherry, adding a few more proteins back into the new whooping cough vaccine could increase their efficacy.

“What we’re seeing is that with kids who are protected by the vaccine, it wears off over time and they’re susceptible again,” Clark says. “We don’t want people to lose confidence in the safety of the new vaccines. We know they protect very well in the short term. We know that people who are vaccinated, even if they get pertussis, they’re very unlikely to require hospitalization. Deaths are almost unheard of in vaccinated people. It’s not that vaccines are failing, they do protect well.”

As a result, the CDC recommends that the best way to protect yourself and your children is to get vaccinated. Those who have already been vaccinated should talk to their doctors about receiving a booster shot, called Tdap.

To get pertussis, you need to be in very close contact with someone; you’re unlikely to pick it up on the bus, unless a person coughs right in your face, since the disease is passed through airborne droplets.

The most vulnerable population when it comes to whooping cough, says Clark, are babies. “That’s where the deaths occur,” he says. “It takes several doses before they’re protected, and they can’t have vaccines until two months. We rely on everybody around babies to be vaccinated to reduce transmission, which is why we’ve recommended a booster for pregnant women.”

Ultimately, the CDC is taking the long view when it comes to the recent outbreak. The agency is working on determining if a new vaccine schedule is necessary to boost immunity, but no new vaccine is in the works—yet. Stay tuned.

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